Field of the Invention
The present invention relates to a process of non-electrolytic copper plating for a printed circuit board. The invention relates particularly to a process of non-electrolytic copper plating for a printed circuit board having a copper plating film with excellent mechanical characteristics.
As is well known, printed circuit boards, which are widely used in various fields of the electronic industry including public-welfare electronic machinery such as radio and television, and high-quality industrial machinery such as electronic computer, information-industry and electronic machinery, have been produced almost all by the etched foil method. In recent years, on the other hand, corresponding to high advances in the electronic machinery industry current trends are toward high density and high performance. When dealing with high density and multilamination of printed circuit boards, the conventional etched foil method is proved to be disadvantageous, because undercuts of films plated on the boards occur in an etching procedure and unequal thickness of films plated on the boards is inevitable. And further, dimensions of patterns formed on the boards are not accurate and overhanging bridge of the patterns can occur. And when small holes in a thick board such as a laminated board, for example, holes of diameter less than 1.00 mm in the board of thickness about 3.2 mm, are plated, thickness of films plated on the corner parts of the holes largely differs from thickness of films plated on the middle parts of the holes and moreover occasionally only the corner parts are plated and the middle parts not plated. Therefore, industrial production of printed circuit boards with high-density wiring and high aspect ratio (board thickness/hole diameter) is difficult. And moreover, according to the etched foil method wherein the copper-clad laminated board is used, following on the printing of a pattern on the laminated board, the plated film, excepting the film plated on the parts on which plated films should be remained must be removed in the etching procedure. The copper wasted in the etching procedure amounts to 50%-80% of the total copper used, which is extremely uneconomical. And further, since such an etching solution used to remove the copper contains, in the main, ferric chloride, cupric chloride and aqueous ammonia, if the solution is discharged carelessly, living environment may be polluted. At any rate, in the etched foil method an economic disadvantage is caused by complexity in the process of production and wastefulness of copper.
Accordingly, in place of the electrolytic plating method an additive method, whereby a pattern and through-holes are formed using non-electrolytic copper plating is attracting special interest. The additive method can meet the requirements of high performance, small size, high reliability and low cost in electronic machinery. According to the method, industrial productions of printed circuit boards having high density wiring and high aspect ratio are thus now practicable.
According to the conventional methods of non-electrolytic copper plating for printed circuit boards, surfaces of a board for plating are first degreased, using an alkaline solution. Then, the surfaces of the board are roughed making use of acids and/or others and an activation threatment is applied. The board is then immersed in a non-electrolytic copper plating bath. By keeping the board in the bath at a predetermined temperature for a specific time, a non-electrolytic copper plating film having a desired thickness is thus obtained. However, the plating film obtained by the conventional method is generally brittle and is deficient in practical use. Disconnection of a pattern formed on the bord and corner crack of through-holes which are formed in the board are caused due to the strain induced by mechanical stress in course of printed circuit board processing and implements fixing. Contrarily when a pattern and through-holes are formed by copper electroplating, there occur no disconnection, stripping and crack of the pattern and corner crack in the through-holes. As a result of measuring mechanical characteristics of a plating film obtained by the copper electroplating, it is known that tensile strength of a plated film was 30 Kg/mm.sup.2 -50 Kg/mm.sup.2, elongation percentage 3%-8% and number of 180.degree. bending times 4.
On the other hand, a plating film obtained by utilizing a non-electrolytic copper plating bath comprising a copper salt, a complexing agent, a reducing agent and a pH modifier possesses tensile stength about 10 Kg/mm.sup.2 -20 Kg/mm.sup.2, elongation percentage about 0.5%, and number of bending times 1. The non-electrolytic copper plating film for the pattern and through-holes of printed circuit boards, therefore, is poor in reliability. Accordingly some attempts have been made to improve mechanical characteristics of the non-electrolytic copper plating film by adding sepecified agents to improve the plating film ductility to the bath, such as an organic sulfur compound like mercaptan, thiol or thio compounds, or heterocyclic compounds like dipyridyl or phenanthroline, and inorganic cyanide compounds. Copper plating films obtained by using such a non-electrolytic copper plating bath possess tensile strength 20 Kg/mm.sup.2 -35 Kg/mm.sup.2, elongation percentage 1%-2% and number of bending times about 2. The plating films are practically utilized for a pattern and through-holes of printed circuit boards. However, the copper plating film for the conductor pattern and through-holes of printed circuit boards with high reliability requirement is required to have mechanical characteristics being equal to those of the copper electroplating film.
Therefore, several chemical agents have been introduced into the non-electrolytic copper plating baths in efforts to improve mechanical characteristics of the non-electrolytic copper plating film. In the additive method using a non-electrolytic copper plating bath adhesive agents are used between the film and a board for absorbing a shock which may happen during the board handling in course of the board processing and implements fixing. However, no adhesive agent effectual in adherence of the non-electrolytic copper plating film to the board in electric insulation or in heat resistance can be found hitherto. And moreover, for the sake of raising the adherence of the non-electrolytic copper plating film, a very harmful acid such as a mixture of chromic and sulfuric acids must be used as a component of the adhesive agent.
Then, according to the Japanese Patent Disclosure No. 57-114657, in forming a non-electrolytic copper plating film on a board, two kinds of non-electrolytic copper plating baths are used: first bath contains a mixture of a copper salt, a complexing agent, a reducing agent and a pH modifier as main basic components, and as a supplementary component an additive agent consisting of at least one selected from the group of nonionic surface active agents of ethylene oxide series, dipyridyl, phenanthroline derivative and cyanide compound; and second bath contains the same basic components as contained in the first bath and as a supplementary component an additive agent consisting of at least one selected from the group of sulfur compounds, silicon compounds and phosphorous compounds. The two kinds of non-electrolytic copper plating baths are used alternately for immersion of a board in the baths, so that the non-electrolytic copper plating film having excellent mechanical characteristics is formed in layers.
However, according to the above-mentioned method, the two kinds of non-electrolytic copper plating baths with different additive agents must be provided separately. And moreover, in the non-electrolytic copper plating baths, micro quantity of the additive agents and considerable quantities of mutually interfering substances are contained. Accordingly, there is the disadvantage that the additive agents difficult to be determined by usual quantitative analysis must be regulated in both the baths for supplement.